New quantitative and functional cell and cytochemical assays are proposed to define and elucidate the megakaryocyte progenitor compartment and its humoral regulation. Studies of a clonogenic cell assay developed in culture for a new class of progenitor capable of proliferation will be extended to examine its relationship to small, acetyl-cholinesterase-positive cells as endoreduplicating progenitors and as a new bioassay for humoral regulators. Combined unit gravity sedimentation and density gradient centrifugation will be used to physically separate and define the cellular characteristics of these progenitors for comparison to maturing megakaryocytes by FMF analysis of DNA content, cell volume and density analysis and extent of proliferative capacity measured in vivo and in vitro. Various antimitotic agents will be used as proliferative probes to examine the relationship of cell mitosis to endoreduplication. Effects of acute and chronic depletion and suppression of platelet production will be studied in experimental mouse model systems. Quantitative changes and recovery in the number of pluripotent stem cells and progenitors and their proliferative state in addition to maturing megakaryocytes and platelets will be measured temporally in response to various degrees of demand. Elucidation of the cellular kinetics of this system will define potential target cells under direct humoral regulation. The role of thrombopoietin and possible platelet chalones will be examined in these models using platelet labeling techniques and clonogenic cell assays to define more precisely their cellular sites and mechanisms of action.